Camera lens positioning using an electro-active device

ABSTRACT

A camera ( 30 ) uses an electro-active device ( 11 ) to position a movable lens. The electro-active device ( 11 ) comprises an electro-active structure in the form of a continuous electro-active member ( 12 ) curving in a helix around a minor axis ( 13 ) which is in itself curved for example in a helix around a major axis ( 14 ). The continuous member ( 12 ) has a bender construction of a plurality of layers ( 21  and  22 ) including at least one layer of electro-active material so that it bends, on activation, around the minor axis ( 13 ). Concomitantly with the bending, the electro-active structure twists around the minor axis. Concomitantly with that twisting, relative displacement of the ends ( 16 ) of the device ( 11 ) occurs due to the combination of the twisting around the minor axis ( 13 ) and the fact that the minor axis ( 13 ) is curved. This structure of the electro-active device ( 11 ) is compact and provides for linear displacement, thereby making it suitable for positioning the lens of the camera ( 30 ).

[0001] The present invention relates to a camera using an electro-activedevice to position a lens.

[0002] Camera lenses need to be moved for focussing and zooming.Otherwise the camera is limited to having a small aperture lens toachieve a sufficient range of focus. Whilst manual adjustment of lensposition is possible, it is desirable to provide for electrically-drivenadjustment.

[0003] To drive camera lenses, it is known to employ an electric-coilmotor. However such motors are expensive and only commonly employed onexpensive cameras. Also, electric-coil motors are relatively bulky andso their use limits miniaturisation.

[0004] Recent developments in CCD and CMOS imaging technology haveallowed the development of small, cheap digital cameras. However veryfew of these cameras have auto-focus systems because current, miniaturelens-positioning systems are too expensive relative to the cost of thecamera.

[0005] Piezoelectric motors for positioning camera lenses are known butthese are expensive. Other known piezoelectric actuators are unable toprovide sufficient displacement in a suitably compact form.

[0006] Electro-active materials are materials which deform in responseto applied electrical conditions or, vice versa, have electricalproperties which change in response to applied deformation. The bestknown and most developed type of electro-active material ispiezoelectric material, but other types of electro-active materialinclude electrostrictive material and piezoresistive material. Manydevices which make use of electro-active properties are known.

[0007] The most simple type of piezoelectric device is a block ofpiezoelectric material activated in an expansion-contraction mode byapplying an activation voltage in the direction of poling. However, asthe piezoelectric effect is small, of the order 10-11 m/V, the change indimensions is relatively small, typically less than a micron. Therefore,more complicated electro-active structures have been developed toachieve larger displacements.

[0008] A known electro-active structure is the bender construction, forexample a bimorph bender construction. With a bender construction, theelectro-active structure comprises a plurality of layers at least one ofwhich is of electro-active material. On activation, the layers deformwith a differential change in length between the layers for example onelayer expanding and another layer contracting. Due to the layers beingconstrained by being coupled to one another, the differential change inlength causes the bender to bend perpendicular to the layers.Accordingly there is a relative displacement of the ends of thestructure. However, the relative displacement does not follow a linearpath in space. As the structure bends and the degree of curvatureincreases, the relative displacement of the ends follows a curve inspace. As a result of this non-linear displacement, such a device isinconvenient for use in positioning a camera lens.

[0009] Furthermore, to achieve relatively large displacement, it isnecessary to increase the length of the structure which thereforebecomes inconvenient. For example, to achieve a displacement of theorder of 0.1 mm with a bimorph bender construction, a structure oflength around 5 cm is typically needed. Thus such a device is notsufficiently compact for use in camera lens positioning.

[0010] According to the present invention, there is provided a camerahaving a movable lens coupled for positioning of the lens to anelectro-active device comprising an electro-active structure extendingalong a curved minor axis and arranged, on activation, for the structureto twist around the minor axis and concomitantly for relativedisplacement of the ends of the structure to occur, the ends of thestructure being coupled to position the lens.

[0011] First, activation of the electro-active device will beconsidered. The relative displacement between the ends of the deviceoccurs concomitantly with the twist of the structure around the minoraxis on activation, because of the fact that the device extends along acurved minor axis. The electro-active device uses the physical principalthat twisting of a curved object causes displacement perpendicular tothe local curve, and vice versa displacement of the ends of a curvedobject causes twisting along its length. The displacement is equivalentto a change in the orientation of the minor axis of the structurerelative to its original orientation.

[0012] The device uses an electro-active structure which twists onactivation. Considering any given small section of the structure alongthe curved minor axis it is easy to visualise how twist of that givensection rotates adjacent sections and hence relatively displaces them inopposite directions perpendicular to the local curve of the givensection, because the adjacent sections extend at an angle to the givensection as a result of the curve of the minor axis. Therefore twistingof the given section is concomitant with a relative displacement of theadjacent sections perpendicular to the plane of the curve. The degree ofrelative displacement is proportional to the degree of curvature in thegiven section and the magnitude of the twisting. The overalldisplacement of the device is the combination of the displacement ofeach section. Thus the overall displacement on activation is a relativedisplacement of the ends of the structure.

[0013] For minor axes which extend along a regular curve around a majoraxis, such as along an arc of a circle or a helix, on activation eachsection produces displacement in the same direction parallel to themajor axis. Therefore, the overall relative displacement of the end ofthe structure is a linear displacement parallel to the major axis.Therefore an electro-active device in accordance to the presentinvention can produce displacement which is linear in space, making itsuitable for positioning of a camera lens.

[0014] The degree of displacement is proportional to the length of thestructure along the minor axis, because each section of the structurecontributes to the overall displacement. Therefore any desired degree ofdisplacement may be achieved by suitable design of the device, inparticular by selection of the length of the structure along the minoraxis and of the type of structure which controls the magnitude of thetwisting-field response. As a result of the structure extending along aminor axis which is curved, a relatively compact device may be produced.In general, the curve along which the minor axis extends may be of anyshape which provides linear displacement.

[0015] One possibility is for the curve along which the minor axisextends to be planar, for example as the arc of a circle or a spiral. Inthis case, the displacement on activation occurs perpendicular to theplane of the curve. The thickness of the device in the direction inwhich relative displacement occurs is merely the thickness of theelectro-active structure so a relatively thin device may be produced.

[0016] Another possibility is for the curve along which the minor axisextends to be a helix. In this case, each helical tun of the structurecontributes towards displacement in the direction along the geometricalmajor axis around which the helix is formed. Therefore a large degree ofdisplacement may be achieved proportional to the number of helicalturns, therefore producing relatively high displacement for a relativelycompact device.

[0017] The electro-active device is by its nature relatively compact.This means it is easily employed in a camera so that the displacement ofthe ends of the structure drives positioning of the lens. Typically thecamera comprises a housing relative to which the lens is moveable andthe ends of the electro-active structure are coupled to the lens and thehousing, respectively.

[0018] In one particularly convenient arrangement, the electro-activestructure is curved around the optical axis of the lens. This provides aparticularly compact arrangement, because it limits the extent to whichthe structure of the electro-active device extends away from the opticalaxis of the lens. Instead, the curve along which the minor axis of thestructure extends, which curve is intrinsic for proper operation of thedevice, can be fitted conveniently around the outside of the lenswithout adding significant bulk to the camera as a whole.

[0019] Use of the electro-active device described above, therefore, candrive positioning of the lens linearly with a relatively compactarrangement. Furthermore, the electro-devices can be produced relativelycheaply compared to other lens positioning systems such as those basedon electro-magnet coil motors, and particularly when compared to knownminiature lens-positioning systems. In addition the electro-activedevice of the present invention inherently provides a relatively highresolution of positional control, the main limiting factor being theresolution of the applied activation voltage.

[0020] The present invention may be applied to lenses which are moveablefor focussing of the camera or for zooming, typically as part of alarger lens arrangement. The present invention is typically applicableto a camera having an electrical image sensor onto which the lensdirects light for production of a digital image, such a charge coupleddevice (CCD) or a CMOS sensor. In this case, the advantages achieved bythe use of the electro-active device compliment the advantages achievedby the use of the electrical image sensor. The output of the imagesensor may be used to control the positioning of the lens. However, thepresent invention is equally applicable to a camera which exposes animage on photographic film.

[0021] Preferably, the electro-active structure of the electro-activedevice comprises electro-active portions disposed successively along theminor axis and arranged to bend, on activation, around the minor axis.

[0022] The electro-active structure is arranged with portions which bendon activation around the minor axis concomitantly with twisting of thestructure around the minor axis. As a result, the electro-active portionmay have any construction which bends on activation. The preferredconstruction is the known bender construction comprising a plurality oflayers including at least one layer of electro-active material,preferably a bimorph bender construction having two layers. Such aconstruction is well known and understood as applied to a straightbender and particularly easy to manufacture. The same benefits areobtained when the bender construction is applied to the portions of thepresent invention. However, any other construction which providesbending on activation may be used.

[0023] Preferably, the electro-active structure comprises a continuouselectro-active member curving around the minor axis, said electro-activeportions being adjacent finite portions of the continuous member.

[0024] This structure is particularly easy to manufacture, for exampleby winding a deformable continuous electro-active member into shape.

[0025] Preferably wherein the continuous electro-active member curves ina helix around the minor axis.

[0026] By using a continuous electro-active member which curves in ahelix around the minor axis a number of advantages are achieved.Firstly, it is easy to provide a structure which is regular along thelength of the minor axis and hence provide the same degree of twistingalong the entire length of the minor axis. Secondly, the helix is easyto manufacture, for example by winding a deformable continuous memberinto shape or by making a helical cut in a tubular electro-activemember. Thirdly, the device is compact as the helical turns of themember around the minor axis may be packed closely together.

[0027] However the electro-active structure may alternatively comprise acontinuous electro-active member having a different shape which providesfor bending around the minor axis concomitantly with twisting around theminor axis. For example it may comprise a continuous member having theshape of a flat member twisted around the minor axis. Furthermore,instead of comprising a continuous electro-active member, theelectro-active structure may comprise a plurality of electro-activeportions coupled together.

[0028] To allow better understanding, embodiments of the presentinvention will now be described by way of non-limitative examples withreference to the accompanying drawings in which:

[0029]FIG. 1 is a plan view of a first electro-active device;

[0030]FIG. 2 is a side view of a second electro-active device;

[0031]FIG. 3 is a perspective view of a portion of either the firstdevice of FIG. 1 or the second device of FIG. 2;

[0032]FIG. 4 is a perspective view of a first camera in accordance withthe present invention;

[0033]FIG. 5 is cross-sectional view of the first camera taken along theline V-V in FIG. 4;

[0034]FIG. 6 is a diagram of the electrical circuits in the first cameraand in a second camera; and

[0035]FIG. 7 is a cross-sectional view of the second camera inaccordance with the present invention.

[0036] According to the present invention, a camera having a moveablelens is provided with an electro-active device for positioning of thelens. For clarity, the electro-active device will first be described,followed by the camera as a whole.

[0037] In the following description, the electro-active devices aredescribed with reference to minor and major axes which are imaginary,but are nonetheless useful for visualising and defining the devices.

[0038] A first electro-active device 1 in accordance with the presentinvention is illustrated in FIG. 1. The device 1 comprises a structureconsisting of a continuous electro-active member 2 curving in a helixaround a minor axis 3 so that the structure extends along the minor axis3. The minor axis 3 is curved, extending in a curve which is an arc of acircle around a geometrical major axis 4 perpendicular to the plane ofthe minor axis 3, i.e out of the plane of the paper in FIG. 1. As theminor curve 3 is planar, the thickness of the device parallel to themajor axis 4 is merely the thickness of the helical structure of theelectro-active member 2.

[0039] A second electro-active device 11 in accordance with the presentinvention is illustrated in FIG. 2. The device 2 comprises a structureconsisting of a continuous electro-active member 12 to curving in ahelix around a minor axis 13 so that the structure extends along theminor axis 13. The minor axis 13 is curved, extending in a curve whichis a helix around a geometrical major axis 14. The electro-active device11 is illustrated in FIG. 2 with a minor axis which extends along of ahelix of three turns merely for illustration, any number of turns beingpossible.

[0040]FIG. 3 illustrates a portion 20 of either the continuous member 2of the first device 1 of FIG. 1 or the continuous member 12 of thesecond device 11 of FIG. 2. The construction of the portion 20 being thesame for both the first device 1 and the second device 2 theelectro-active portion 20 is a finite portion of the continuous member 2or 12 and hence the electro-active member 2 or 12 may be considered as aplurality of adjacent portions 20 as illustrated in FIG. 3 disposedsuccessively along the minor axis 3 or 13. Hence, the portion 20 extendsalong part of a helical curve around the minor axis 3 or 13 as shown inFIG. 3.

[0041]FIG. 3 illustrates the construction of the electro-active portion20. This construction is preferably uniform along the entire length ofthe minor axis 3 or 13 in order to provide uniform properties onactivation. Alternatively, the device 1 or 11 may be designed with somevariation along the length of the minor axis 3 or 13, either in theconstruction of the continuous member 2 or 20 or in the shape of thecurve of the continuous member 2 or 20 around the minor axis 3 or 13.

[0042] The electro-active portion 20 has a bimorph bender constructioncomprising two layers 21, 22 of electro-active material extending alongthe length of the portion 20. The layers 21, 22 of electro-activematerial both face the minor axis 3 or 13. The electro-active layers 21or 22 preferably extend, across the width of the portion 20, parallel tothe minor axis 3 or 13, although there may be some distortion of theelectro-active portion 20 of the continuous member 2 or 12 due to thenature of the curve around the minor axis 3 or 13. Alternatively, thelayers 21 or 22 may extend, across the width of the portion 20, at anangle to the minor axis 3 or 13 so that one edge along theelectro-active portion 20 is closer to the minor axis 3 or 13 than theopposite edge.

[0043] The material of the electro-active layers 21 or 22 is preferablypiezoelectric material. The piezoelectric material may be any suitablematerial, for example a piezoelectric ceramic such as lead zirconatetitanate (PZT) or a piezoelectric polymer such as polyvinylidenefluoride(PVDF). However, the material of the electro-active layers 21, 22 may beany other type of electro-active material, for example piezoresistivematerial, in which the electrical resistance changes as the material isdeformed or strained, or electrostrictive material, which constricts onapplication of an electric field.

[0044] The electro-active portion 20 further comprises electrodes 23 to25 extending parallel to the layers 21, 22 of piezoelectric material.Outer electrodes 23, 24 are provided outside the electro-active layers21, 22 on opposite sides of the electric-active portion 20. A centreelectrode 25 is provided between the electro-active layers 21 and 22.The electrodes 23 to 25 are used to apply poling voltages and to operateelectro-active portion 20 in a bending mode. On electrical activation,activation voltages are applied to the electrodes 23 to 25. Onactivation, the electro-active layers 21 and 22 undergo a differentialchange in length concomitant with bending of the portion 20 due to theconstraint of the layers being coupled together at their interfaceformed by the centre electrode 25. For maximum displacement, onactivation one of the electro-active layers 21 or 22 expands and theother one of the electro-active layers 21 and 22 contracts. The relativedirection and magnitude of the activation and poling voltages may beselected in the same manner as for known linear electro-active deviceshaving a bender construction. For example, poling voltages of sufficientmagnitude to pole the electro-active layers 21 and 22 may be applied inopposite directions across the electro-active layers 21 and 22 bygrounding the centre electrode 25 and applying poling voltages of thesame polarity to both the outer electrodes 23, 24. In this case, theelectro-active portion 20 is electrically activated by applyingactivation voltages in the same direction across the electro-activelayers 21 and 22 by applying voltages of opposite polarity to the twoouter electrodes 23 and 24.

[0045] On activation the electro-active portion 20 bends around theminor axis 3 or 13, either towards or away from the minor axis 3, 13depending on the polarity of the activation voltages. On electricalactivation the activation voltages are applied from a circuit 26 throughexternal terminals 27 electrically connected to the electrodes 23 to 25in the manner known for known straight piezoelectric devices having abender construction.

[0046] Electrical connection to the electrodes 23 to 25 may be made inthe same way as is known for known straight devices having a benderconstruction, in principle at any point along the length of the deviceof which the portion 20 forms part but preferably at the end. Thepreferred technique is to provide the electrodes with fingers (notshown) extending at the end of the device at different lateral positionsacross the width of the device as known for straight devices having abender construction.

[0047] It will be appreciated that other bender constructions couldequally be applied to the portion 20, for example a unimorph benderconstruction comprising a layer of electro-active material and aninactive layer or a multimorph bender construction comprising aplurality of layers of electro-active material.

[0048] Whilst the bender construction illustrated in FIG. 3 is preferredfor simplicity and ease of manufacture, it will be appreciated that thecontinuous numbers 2 or 12 could in fact have any construction whichbends around the minor axis 3 or 13 on activation. For example, thecontinuous members could be electro-active elements of the typedescribed in the application being filed simultaneously with thisapplication entitled “Electro-Active Elements and Devices” in which theelements have two pairs of electrodes extending along the length of themember for bending across the width on activation.

[0049] On activation, the electro-active portions 20 of the continuousmember 2 or 12 bend around the minor axis 3 or 13. As a result of thecontinuous electro-active member 2 or 12 curving around the minor axis 3or 13, in particular in a helix, such bending is concomitant withtwisting of the continuous member 2 or 12 around the minor axis 3 or 13.This may be visualised as the turns of the continuous member 2 or 12 asthe bending tightening or loosening causing a twist of the structure ofthe member 2 or 12 along the minor axis 3 or 13. The twist of thecontinuous member 2 or 12 occurs along the entire length of the minoraxis 3 or 13 causing a relative rotation of the ends of the structurelabelled 5 and 6 in the first device 1 of FIGS. 1 and 15 and 16 in thesecond device 11 of FIG. 2.

[0050] It will be appreciated that the continuous member 2 or 12 couldcurve around the minor axis 3 or 13 in curves other than a helix toproduce such twisting, for example by having the shape as though formedby twisting a flat member round the minor axis. It will also beappreciated that other structures other than a continuous member couldbe applied to produce twisting around the minor axis. For example theelectro-active structure could consist of a plurality of electro-activeportion disposed successively along the minor axis and coupled togetherso that the bending of each individual portion twists the adjacentportion around the minor axis causing twisting of the structure as awhole. Alternatively the electro-active structure could be a device ofthe type described in the application being filed simultaneously withthis application entitled “Piezoelectric Devices” which comprises aplurality of electro-active torsional actuators which may compriseelectro-active elements activated in shear mode.

[0051] Considering the first device 1 of FIG. 1, the twisting of thecontinuous member 2 around the minor axis 3 is concomitant with relativedisplacement of the ends of the device 5 and 6 perpendicular to thecurve of the minor axis 3, that is parallel to the major axis 4. Therelative displacement of the ends 5 and 6 derives from the twisting ofthe continuous member 2 around the minor axis 3 in combination with thecurve of the minor axis 3. It is an inevitable result that twisting of acurved object causes relative displacement of the ends of that objectperpendicular to the local curve of the object.

[0052] In a similar manner, on activation of the second device of FIG.2, the twisting of the continuous member 12 around the minor axis 13 isconcomitant with displacement of the ends of the device 15 and 16parallel to the major axis 14. Again, this relative displacement derivesfrom the rotation of the continuous member 12 around the minor axis 13in combination with the curve of the minor axis 13. In this case, therelative displacement caused by any given small section of the structurealong the minor axis 13 causes relative displacement of the ends of thatsection perpendicular to the local curve of the minor axis 13. Theoverall displacement of the ends 15, 16 of the device 11 is the sum ofthe displacements of all the sections which results in an overallrelative displacement parallel to the major axis 14.

[0053] The exact construction and dimensions of the member 2 or 12 andthe form of the electro-active structure may be freely varied to producethe desired response. A suitable member 2 or 12 has a 0.5 mm thicknesstape wound as a 4 mm diameter minor helix around the minor axis 3 or 13.When this forms the first device 1 in which the minor curve extendsaround about three quarters of a circle of 30 mm diameter the observeddisplacement is about ±6 mm. Similarly if this structure was used toform the second device 11 in which the minor curve extends along a 20turn helix of diameter 30 mm, this would produce displacement of around±120 mm.

[0054] In general, the minor axis, along which the structure of devicesin accordance with the present invention extend, may follow any curveand the resultant displacement of the ends of the structure will be thesum of the displacement caused by each section of the structure alongthe curve. However, curves which are regular such as the curve of theminor axis of the first and second devices 1 and 11 are preferred sothat all sections of the device caused relative displacement in a commondirection and also because design and manufacture are therebysimplified.

[0055] In accordance with the present invention the first and seconddevices 1 and 11 are electrically activated to create mechanicaldisplacement between the ends 5 and 6 or 15 and 16, although the devices1 and 11 are capable of being mechanically activated in which caserelative displacement of the ends 5 and 6 or 15 and 16 causes anelectrical voltage to be developed across the electrodes 23 to 25.

[0056] Manufacture of the electro-active devices 1 and 11 will now bedescribed.

[0057] The preferred method of manufacture is to initially form theelectro-active structure extending along a straight minor axis andsubsequently to bend the straight electro-active structure so that theminor axis along which it extends becomes curved.

[0058] To form the continuous member 2 or 12 as an electro-activestructure along a straight minor axis there are two preferredtechniques.

[0059] The first preferred technique is to initially form the continuousmember 2 or 12 as a straight member and subsequently to deform it tocurve around the straight minor axis. The bender construction of thecontinuous member 2 or 12 is in itself known and the continuous member 2or 12 may be formed by applying any of the known techniques formanufacturing a device having a bender construction. For example, thecontinuous member 12 may be initially manufactured by co-extrusion ofthe layers 21 and 22 of plasticised material or by co-calendering of thelayers 21 and 22. Alternatively, the continuous member 2 or 12 may bemade through lamination of thin layers 21 and 22. These thinner layersmay be made by any suitable route, such as high shear mixing of aceramic powder, polymer and solvent mixer followed by co-extrusion andcalendering. Alternatively, techniques such as tape casting or theprocess known as the Solutech process known in the field of ceramics maybe used.

[0060] The electrodes may be formed as an integral part of themanufacture of the continuous member 2 or 12, for example by being inco-extruded or co-calendered. Further electrodes, which may beactivation layers 23 to 25 or may be terminal electrodes to allow accessto the electrodes 23 to 25, may be applied by printing, by electro-lessplating, through fired-on silver past or by any other appropriatetechnique.

[0061] The second preferred technique is to initially manufacture thecontinuous member as a cylinder or other tube with a multi-layeredbender construction of electro-active layers 21 and 22 and electrodes 23to 25 and subsequently to cut the member along the helical line to leavethe continuous member 2 or 12 extending in a helix around the axis ofthe cylinder or tube which then constitutes the minor axis.

[0062] Subsequently the straight structure is bent to curve the minoraxis along which the structure extends.

[0063] To deform the member and structure, there must exist in theinitially formed member a sufficient degree of flexibility. Suitablydeformable electro-active materials are known, typically includingconstituent polymers which enhance the deformability. With suchmaterials after shaping, the constituent polymers are burnt out,typically at up to 600° C. and the material is then densified throughfurther sintering at higher temperature, typically 1000° C. to 1200° C.In this case, the electro-active structure is initially formed withenlarged dimensions to allow for linear shrinkage which occurs duringsintering, typically of around 12 to 25%.

[0064] The curving of the straight member and the bending of thestructure may be performed around formers. The formers are subsequentlyremoved either physically or by destruction of the former for example bymelting, burning or dissolving.

[0065] Cameras using the electro-active devices of the type describedabove will now be described. The electro-active devices are describedand illustrated as having the same structure as the first device 1described above extending around an arc of a circle, but this is merelyfor illustration as the electro-active devices may have any of the typesof structure described above. However, it is preferable to use anelectro-active device in which the curved minor axis along which thestructure of the device extends is planar, such as the firstelectro-active device 1, from the point of view of reducing thethickness of the camera 30.

[0066] A first camera 30 in accordance with the present invention isillustrated in FIGS. 4 and 5. The camera 30 comprises a housing 31having an electrical image sensor 32 mounted to the rear face 33 of thehousing 31 inside the housing 31. The image sensor 32 may be any type ofimage sensor, such as a charge coupled device (CCD) or a CMOS sensor.

[0067] In the front face 34 of the housing 31, there is mounted a lensunit 35 which comprises a lens 36 fixed inside a cylindrical carrier 37.The lens 36 directs light along its optical axis 38 to form an image onthe image sensor 32. The carrier 37 is slidably mounted in the frontface 34 of the housing 31 to allow movement of the lens 36 parallel toits optical axis 38. Such movement of the lens 36 allows it to bepositioned to focus the image formed by the lens 36 on the image sensor32.

[0068] To drive movement of the lens 36, an electro-active device 40 ofthe type described above is coupled in between the housing 31 and thelens 36. The electro-active device 40 is arranged so that the minor axisalong which the structure of the electro-active device 40 extends iscurved around the optical axis 38 of the lens 36, preferably lying in aplane which is perpendicular to the optical axis 38. As a result, onactivation, the ends 41 and 42 of the device 40 are relatively displacedparallel to the optical axis 38. One end 41 of the electro-active device40 is coupled to the housing 31, whereas the other end 42 is coupled tothe carrier 37 of the lens unit 35, and hence is indirectly coupled tothe lens 36. Accordingly, activation of the electro-active device 40drives movement of the lens 36 parallel to its optical axis 38 to adjustthe focussing of the camera

[0069] The circuitry of the first camera 30 is illustrated in FIG. 6.The circuitry includes a control circuit 45 powered by a battery 46. Thecontrol circuit 45 receives and processes the output of the image sensor32. The control circuit 45 also controls the operation of the camera 30in a conventional manner and may have any suitable form for thispurpose, typically being a microprocessor. In addition to thesefunctions, the control circuit 45 is connected to the electrodes 23, 24of the electro-active device 40 and supplies activation voltages toactivate it. The magnitude of the applied activation voltages controlsthe magnitude of displacement of the ends 41 and 42 of the device 40,hence controlling the position of the lens 36 along the optical axis 38and the focussing of the camera 30.

[0070] The control circuit 45 may control the position of the lens 36 inresponse to manually operated buttons 47 provided on the housing 31 ofthe camera 30, to allow a user to indicate the desired degree of focus.Alternatively, the control circuit 45 may control the position of thelens 36 in response to the output of the image sensor 32. In this casethe control circuit 45 detects the degree of focus from the output imagesignal and controls the position of the lens to automatically focus theimage. To do this, the control circuit 45 uses any of the knownauto-focus algorithms, for example by maximizing the contrast in theimage output from the image sensor 32.

[0071] A second camera in accordance with the present invention isillustrated in FIG. 7. Except for the differences described below, thesecond camera 50 is identical to the first camera 30 and so for thecommon elements the same reference numerals will be used and adescription thereof will not be repeated.

[0072] The second camera 50 differs from the first camera 30 by having acompound lens system. In particular the second camera 50 is additionallyprovided with a second lens unit 51 comprising a second lens 52 fixedinside a second carrier 53 which is slidably mounted inside the firstmentioned carrier 37 of the first mentioned lens unit 35. Thus thesecond lens 52 is moveable coaxially with the first lens 36 along theoptical axis 38. The relative position of the second lens 52 withrespect to the first lens 36 alters the magnification of the lens systemformed by the two lenses 36 and 52 together, thereby controllingzooming. To drive such movement of the second lens 52 with respect tothe first lens 36, a second electro-active device 54 of the typedescribed above is coupled between the first and second lens units 35and 51, hence being indirectly coupled to the first and second lenses 36and 52. Activation of the second electro-active device 54 thereforecontrols the position of the second lens 52 with respect to the firstlens 36 and hence controls zooming of the camera 50.

[0073] The electrical circuit illustrated in FIG. 6 is also used in thesecond camera 50. In the same way as the first camera 30, the controlcircuit 45 applies activation voltages to the electrodes 23, 24 of thesecond electro-active device 54, either in response to manual controleffected by the user or by using optical feedback responsive to theoutput from the image sensor 32.

[0074] In general, an electro-active element of the type described abovemay move any lens of a camera, whether it is the sole lens as in thefirst camera 30 or a lens of the compound lens system as in the secondcamera 50. The element may move the lens relative to the housing of thecamera or relative to the other lenses of a compound lens system, orboth.

1. A camera having a movable lens coupled for positioning of the lens toan electro-active device comprising an electro-active structureextending along a curved minor axis and arranged, on activation, for thestructure to twist around the minor axis and concomitantly for relativedisplacement of the ends of the structure to occur, the ends of thestructure being coupled to position the lens.
 2. A camera according toclaim 1, wherein the electro-active structure comprises electro-activeportions disposed successively along the minor axis and arranged tobend, on activation, around the minor axis.
 3. A camera as claimed inclaim 2, wherein the electro-active structure comprises a continuouselectro-active member curving around the minor axis, said electro-activeportions being adjacent finite portions of the continuous member.
 4. Acamera as claimed in claim 3, wherein the continuous electro-activemember curves in a helix around the minor axis.
 5. A camera as claimedin any one of claims 2 to 4, wherein the successive electro-activeportions have a bender construction of a plurality of layers includingat least one layer of electro-active material.
 6. A camera as claimed inclaim 5, wherein the electro-active portions have a bimorph benderconstruction of two layers of electro-active material or a multimorphbender construction of more than two layers of electro-active material.7. A camera as claimed in any one of the preceding claims, wherein theelectro-active structure includes electrodes for application of anelectric field to activate the electro-active structure.
 8. A camera asclaimed in any one of the preceding claims, wherein the minor axisextends in curve which is a helix.
 9. A camera as claimed in any one ofthe preceding claims, wherein the minor axis extends in a curve which isplanar.
 10. A camera as claimed in any one of the preceding claims,wherein the electro-active structure includes piezoelectric material.11. A camera as claimed in claim 10, wherein the piezoelectric materialis a piezoelectric ceramic or a piezoelectric polymer.
 12. A camera asclaimed in claim 11, wherein the piezoelectric material is leadzirconate titanate (PZT) or polyvinylidenefluoride (PVDF).
 13. A cameraas claimed in any one of the preceding claims, wherein the cameracomprises a housing relative to which the lens is movable, the ends ofthe electro-active structure being coupled to the lens and the housing,respectively.
 14. A camera as claimed in any one of the precedingclaims, wherein the lens is movable for focussing of the camera.
 15. Acamera as claimed in any one of the preceding claims, wherein the lensis movable for zooming.
 16. A camera as claimed in any one of thepreceding claims, wherein the lens is part of a compound lens system.17. A camera as claimed in any one of the preceding claims, furthercomprising a control circuit for activating the electro-active device toposition the lens.
 18. A camera as claimed in any one of the precedingclaims, wherein the camera further comprises an electrical image sensoronto which the lens directs light.
 19. A camera as claimed in claim 18when appendant to claim 17, wherein the control circuit is responsive tothe output of the image sensor to control the positioning of the lens.20. A camera as claimed in any one of the preceding claims, wherein theelectro-active structure extends along a minor axis which is curvedaround the optical axis of the lens.